1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same. More particularly, the present invention relates to a quantum dot semiconductor device and a method for manufacturing the same.
2. Description of the Related Art
Recently, a technology of using quantum dots for a gain medium is proposed for high-performance of optical communication devices. Such a device is expected to be applied particularly to a semiconductor amplifier using a broad band property due to inhomogeneous broadening of quantum dots, which serves as a repeater of a wavelength multiplexing communication system.
To employ quantum dots for a semiconductor device, it is required that for a light with a variable polarization direction such as a light passing through an optical fiber, sufficient gains are secured without depending on polarization of a signal light. Therefore, in order to obtain a quantum dot structure securing sufficient gains without depending on polarization of a light, a height of the quantum dot must be set to the same size as that in the horizontal direction of the quantum dot. For one of such shapes, there has been proposed a columnar quantum dot in which a plurality of flat-shaped quantum dots self-formed by the Stranski-Krastanov (S-K) growth mode are stacked almost at intervals of being coupled quantum mechanically.
However, it is confirmed that when columnar quantum dots are stacked by a spacer layer with a film thickness of about 40 nm capable of stacking of ordinary quantum dots, crystallinity deteriorates and therefore, photoluminescence intensity decreases. Deterioration of the crystallinity is caused by the following factor. That is, the columnar quantum dots undergo compressive strains so as to achieve a lattice matching with a substrate. Further, the strains are accumulated in proportion to the stacking number. As a result, a thickness of the accumulated strains is excessively increased to exceed a film thickness capable of growth (critical film thickness) and therefore, strain relaxation with generation of dislocation is generated in a crystal.
Accordingly, a quantum dot having a structure as shown in the following FIG. 6 is proposed as a method for preventing deterioration of the crystallinity due to the stacking of quantum dots (see, e.g., Japanese Unexamined Patent Application Publication No. 2003-197900). FIG. 6 is a constitution diagram of a columnar quantum dot. A columnar quantum dot 400 of FIG. 6 employs a strain compensation structure in which on a substrate 402, a quantum dot layer 401 and a barrier layer 420 made of materials with a strain property are alternately stacked several times. The strain compensation structure is proposed for a general columnar quantum dot.
However, when the strain compensation structure is applied, the barrier layer made of materials with a strain property covers the quantum dot also at the part contacting the quantum dot. As a result, the barrier layer compensates residual strains in the whole crystal as well as changes a local strain distribution within the quantum dot. The local strains within the quantum dot are a factor for determining a polarization characteristic and a photoluminescence wavelength. Therefore, there is a problem that a polarization characteristic and a polarization wavelength are inappropriately changed due to change of the strain distribution within the quantum dots.